Make p subject of the formula given that M=PK/MK+P
M = PK/(MK+P) cross-multiply
M(MK + P) = PK multiply out the bracket
M^2.K + PM = PK collect common terms on rhs
M^2.K = PK - PM tale out common factor
M^2.K = P(K - M) make P the subject
P = M^2.K/(K - M)